It's a pun, but kind of hard to explain. Basically, it's because termites eat wood, and the bar is made of wood. A 'bartender' is someone who works behind a bar, but in this case, the joke is that the termite is asking if the "bar" is "tender" (i.e., nice to eat).

Yes. Termites eat things that are made of wood and bars are typically made of wood. In this joke, tender means 'Does it taste good?'. But a bartender is someone who brings people drinks and things in a resturaunt that work behind the bar. So the termite is asking if the bar is tender, or good to eat.

Please delete

I'm glad Aerie explained it. That's funny! ^_^

Yes, that is the beginning of a joke. You won't get it until you hear the punch-line (the end of the joke). I have not heard the joke, so I don't know the funny part. Have you heard the rest of the joke?